1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery or cell for use as power source of portable electronic devices, and a method for producing cathode material of the non-aqueous electrolyte secondary battery.
2. Prior Art
In association with a recent remarkable progress of electronic techniques, various portable electronic devices require development of a high-power and small-size secondary battery or cell. There are widely known conventional secondary battery such as nickel-cadmium secondary battery, lead accumulator nickel-hydrogen cells, lithium ion secondary battery, and others. In particular, the lithium ion secondary battery exhibits a high voltage, a high energy density, least self discharge, and an excellent cycle property, and is the most hopeful battery for reducing the size and weight of the battery.
For such lithium ion secondary cells, research has been made for cathode material such as LiCoO.sub.2, LiNiO.sub.2, and a lithium-manganese oxide of a lower cost such as LiMn.sub.2 O.sub.4.
However, the lithium-manganese oxide in a fine powder state which has been conventionally used as cathode material cannot be filled sufficiently dense by mechanical compression alone. Especially when the cathode material is formed into a sheet-type electrode, because of the powder characteristics, it is difficult to obtain a large-capacity cell having a flexibility. That is, such powder cannot serve as a material for practical electrodes. Moreover, there is a problem that the lithium ion secondary battery using this fine powder of lithium-manganese oxide as an cathode material exhibits a cycle property which is significantly lowered after several tens of times of charge/discharge. The charge/discharge property is also significantly lowered by lithium in/out movement. Thus, it is difficult to obtain a battery of a high capacity and high power by using a lithium-manganese oxide in a fine powder state.
Furthermore, when using a lithium-manganese oxide having a large particle size which is made from an electrolytic manganese dioxide or the like, because of its small specific surface, it is necessary to mix it with 10% or more of an electroconductive reagent such as fine graphite and acetylene black so as to increase the number of contact points for increasing electron conductivity. However, even if 10% or more of an electroconductive reagent is mixed, the material changes its properties as the cycle proceeds and the discharge capacity is gradually lowered. Moreover, when an electroconductive reagent and metal are added in a great amount for maintaining the charge/discharge property of the active material, this will increase the battery size and cannot realize the simultaneous demand for a higher power and a larger capacity (small size).
Thus, the conventional lithium-manganese oxide which has been used as an cathode material has various problems. When the particle size is small, the cathode filling density cannot be homogeneous or may be lowered and the electrode lacks in flexibility, which affects the cycle property and the capacity. On the contrary, when the particle size is great, a great amount of electroconductive material is required, disabling to increase the specific capacity. Consequently, in spite of the theoretical capacity of 148 mAh/g in LiMn.sub.2 O.sub.4, the conventional lithium-manganese oxide practically exhibits a charge/discharge capacity of only 110 mAh/g and a cycle service life of 100 cycles. Thus, in practice, only 80% of the theoretical value can be obtained at the most.
In order to solve these problems, research has been made as for the composition of the lithium-manganese oxide and the method for preparing the oxide. However, the lithium-manganese oxide decreases its reversibility as charge/discharge proceeds, significantly lowering its capacity. Thus, the lithium-manganese oxide practically cannot be used as the cathode material. Furthermore, the lithium-manganese oxide is inferior to a lithium-cobalt oxide or lithium-nickel oxide in charge/discharge property at a great current.
In order to solve the above-mentioned problems, the inventors of the present invention examined a crystalline structure which carries out smooth insert and separation of lithium ions. As a result, we have reached the present invention.